Process for the production of multilayer polymer films

ABSTRACT

A process for the production of a composite film for use in laminated glazing assemblies comprising at least one layer of a flexible safety film together with one layer of flexible plastic carrier film includes the steps of drawing the layers from separated feed rolls ( 1, 2 ), heating to a bonding temperature, and pressing together to form a composite film, whereby the flexible safety film is not under tension when it is pressed together with the carrier film to form the composite film.

DESCRIPTION

[0001] This invention relates to novel processes of the production ofcomposite polymer films for use in laminated glazing assemblies andespecially in glazings for use in automotives, to the films producedusing such processes and to laminated glazings incorporating thosefilms.

[0002] There is an increasing demand for glazings especially automotiveglazing which combine solar reflecting properties with the appropriatesafety properties. One type of product which is used to meet this demandcomprises a laminated glazing having an interlayer which comprises aflexible plastic safety film such as polyvinylbutyral (hereinafter forconvenience PVB) together with a flexible plastic carrier layer whichhas a solar control coating layer upon at least one of its surfaces. Onetypical flexible carrier material is polyethylene terephthalate(hereinafter for convenience PET).

[0003] Laminated glazing comprising such an interlayer may be producedby taking a first sheet of glass, the bottom sheet and placingsuccessively on top of that sheet a sheet of flexible plastic safetyfilm; a sheet of flexible plastic carrier film; a further sheet offlexible plastic safety film and a second glass sheet, the top sheet.The resulting stack may be formed into a laminated glazing usingconventional techniques. However an operation of the type describedabove is time consuming and laborious and also increases thedifficulties of removing air bubbles trapped between the various layersof the stack. For this reason it has been proposed, far example in U.S.Pat No. 4,973,511, to form a composite pre-laminated film comprising atleast one layer of safety film and at least one layer of coated carrierfilm. Typically a layer of safety film and a layer of carrier film canbe prelaminated to form a bilayer or a further layer safety film may beintroduced to form a trilayer in which the carrier film is encapsulatedby the two separate layers of safety film. International PatentApplication WO 00/46027 describes a lamination machine useful in theproduction of a PVB/PET/PVB trilayer material which is free of wrinkles.In use the machine draws the PVB under tension through a heating zonewhere it is heated to a temperature at which it will bond to the PET andthrough a set of nip rollers where the PVB and PET films are laminatedto form a trilayer material. The use of such composite pre-laminatedfilms simplifies the production of the laminated glazing but can lead toother problems. The laminated glazings may have a wrinkled appearance,possibly in specific localised areas, which is not acceptable. Aspecific example occurs when a part of the carrier film is removed toprovide a window in the solar control coating. Such a window may beintroduced to provide a telepass area through which electromagneticradiation may pass. Such glazings suffer from an increased tendency ofthe laminated glazing to have a wrinkled appearance especially in theproximity of the window in the solar control coating.

[0004] We have now discovered that composite films having a reducedtendency to wrinkle when subjected to a lamination process may beproduced by allowing the PVB safety films to relax prior toincorporating them into a composite film. Conventional methods for theproduction of these composite films such as that illustrated in FIG. 5of U.S. Pat. No. 4,973,511 and FIG. 1 of WO 00/46027 maintain the PVBunder tension whilst it is passed from a feed roll to a nip roll whereit is bonded with a PET film to form a composite film. In the processesof this invention the safety film is allowed to relax prior to drawingit through the nip roll whilst it is not under tension.

[0005] Accordingly from the first aspect this invention provides aprocess for the production of a composite film comprising at least onelayer of a flexible safety film together with at least one layer of aflexible plastic carrier film wherein said layers are drawn fromseparate feed rollers, heated to a bonding temperature and pressedtogether to form a composite film which process is characterised in thatthe flexible safety film is not significantly stretched prior to itsbeing incorporated into the composite film. In the preferred embodimentsthe flexible safety film is heated to a temperature which is higher thanthe bonding temperature prior to its being incorporated into thecomposite film.

[0006] Any of the safety films known to be useful in the production ofconventional laminated windshields are potentially useful in the processof this invention. Polyurethanes, polyvinyl chlorides, polyvinylacetaland ethylene vinyl acetate are all potentially useful but the mostwidely used safety film is plasticised polyvinyl butyral (PVB). If onlyfor reasons of its ready availability PVB is the preferred safety filmto use in the processes of the invention.

[0007] The thickness of the safety film will vary according to therequirements or the design. Generally each layer of PVB will be at least100 microns thick and usually no more than 2000 microns. The thicknesswill vary with the number of layers of PVB which are incorporated intothe composite film. In the preferred embodiment wherein the compositefilm comprises a single layer of flexible plastic carrier encapsulatedbetween two layers of safety film the combined thickness of the layersof safety film will typically be in the range 500 to 1000 microns. Mostcommonly the two layers of safety film will be of approximately equalthickness.

[0008] The flexible plastic carrier layer may also be formed from avariety of known materials such as PET, nylons, polyurethanes,polyacrylics, polycarbonates, polyolefins, cellulose acetate andpolyvinyl chloride. The preferred material is PET.

[0009] The thickness of the flexible plastic carrier layer is notcritical and will generally be in the range 25 to 10 microns. The PETmay be provided with an infra red reflecting coating prior to theformation of the composite material or after the formation of thecomposite material. Suitable reflecting coatings may be deposited usinga variety of techniques and materials. A preferred form of coating is astack comprising alternative layers of a dielectric and a metal.Examples of suitable metals are silver, gold, palladium, chromium,aluminium, nickel, copper and steel. Examples of useful dielectricsinclude tin oxide, indium tin oxide, titanium oxide, zinc oxide,aluminium oxide, tungsten oxide, tantalum oxide and zirconium oxide.

[0010] The composite films of the type described above mayconventionally be produced by heating at least one safety film and atleast one carrier film to a temperature at which at least one of thosefilms, normally the safety film, become tacky and pressing the filmstogether to form a composite film which may conveniently be achieved bypassing the films through a nip roller. The nip roller is adjusted toexert a sufficient pressure as to cause the films to bond together. Thisbonding need only be sufficiently strong to produce a composite filmwhich can be handled and cut. The composite film can be used in aconventional laminating system. The safety film and the carrier filmbecome strongly bonded during this lamination process.

[0011] In a preferred embodiment of the processes of this invention thesafety film (or films) is heated to an elevated temperature during thatpart of the process when it is not under tension. Conveniently the filmwill be heated to a temperature slightly higher than that which isdesired at the point where it is pressed to form the composite film.Typically a PVB safety film is pressed at a temperature in the range 60°C. to 70° C. In those preferred embodiments the PVB will preferably beheated to a temperature in the range 70° C. to 80° C. whilst not undertension and will cool by natural convection as it passes to the niprollers. This heating of the safety film assists in the relaxation ofthe film and results in the production of composite films having areduced tendency to wrinkle during the lamination process.

[0012] A typical prior art process is illustrated in FIG. 5 of U.S. Pat.No. 4,973,511 the relevant disclosures of which are hereby incorporatedby reference. The process illustrated describes the production of atrilayer comprising a single layer of a carrier film (PET) encapsulatedbetween two layers of a safety film (PVB). The PVB is taken from feedrollers 54 a and 54 b under tension to nip rollers 52 via tension roll55. This results in a significant stretching of the PVB.

[0013] The preferred embodiment of the process for the production of acomposite film which is a bilayer formed by pressing a PVB safety filmand a PET carrier film is illustrated diagrammatically in FIG. 1. InFIG. 1 the numeral 1 represents a feed roller for the PVB film. 2represents a feed roller for the PET film. 3 represents an infra redheater. 4 and 5 represent pinch rollers. 6 represents a spiral roller. 7and 8 represent nip rollers. 9, 14 and 15 represent idler rollers and10, 11, 12 and 13 represent cooling rollers. In use PVB is drawn fromfeed roller 1 by the pinch rollers 4. The temperature of the PVB israised as it passes in front of infra red heater 3. The hot film passesaver spiral roller 6 which is operated so as to avoid any stretching ofthe PVB during its passage from pinch rollers 4 and 5 to spiral roller6. The heated PVB film passes to nip rollers 7 and 8.

[0014] The PET film is drawn from feed roller 2 and passes to niprollers 7 and 8. The PVB and PET films are pressed together between therollers 7 and 8 to form a composite film. The composite film passes overidler roller 9 and cooling rollers 10, 11, 12 and 13 before being drawnover idler rollers 14 and 15 by the action of collecting roller 16.

[0015]FIG. 2 illustrates a second preferred embodiment of the inventionwhich is a process for the production or a trilayer composite film whichis formed by pressing a single layer or PET carrier film between twofilms of a PVB safety film. The elements illustrated denoted by thenumerals 1 to 16 are identical with those described in relation toFIG. 1. The processes illustrated in FIG. 2 additionally require theprovision of a second feed roller 17, a pair of pinch rollers 18 and 19,an infra red heater 20 and a spiral roll 21. The second film of PVB isdrawn from feed roller 17 by the action of pinch rollers 18 and 19. Thetemperature of the PVB film is raised as it passes in front of infra redheater 20. The hot film passes aver spiral roller 21 and is fed to niprollers 7 and 8 The spiral roller 21 is operated so as to avoid anystretching of the PVB during its passage from pinch rollers 18 and 19 tothe nip rollers 7 and 8.

[0016] The speed of operation of the various rollers is adjusted so asto ensure that the PVB is not under tension as it passes from pinchrollers 4 and 5 to the spiral rollers 6 and 21. The heaters 3 and 20 areoperated so as to provide that the temperature of the PVB is in therange 70° C. to 80° C. when it reaches the spiral rollers 6 and 21.

[0017] The composite film is wound on collecting roller 16. In apreferred embodiment an interleaving film which may be a polyethylenefilm may be incorporated onto one surface of the composite film. Theprovision of such an interleaving layer is particularly valuable whenthe composite film is a trilayer composite film. The interleaving filmprevents the PVB surfaces from becoming bonded together.

[0018] The composite films produced by the processes of the inventionare useful in the production of laminated glazings especially laminatedwindscreens for motor vehicles. The composite film is sandwiched betweena pair of glass sheets and any excess of film trimmed from the edges ofthe glass. The film is then degassed using conventional techniques suchas nip rolling or vacuum degassing before being heated in an autoclaveto produce laminated glazing. The films produced by the processes ofthis invention have a reduced tendency to wrinkle during this laminationprocess compared to conventional film.

1. A process for the production of a composite film comprising at leastone layer of a flexible safety film together with at least one layer offlexible plastic carrier film wherein said layers are drawn formseparate feed rollers, heated to a bonding temperature and pressedtogether to form a composite film which is characterised in that theflexible safety film is not under tension when it is pressed togetherwith the carrier film to form the composite film.
 2. A process accordingto claim 1 characterised in that the flexible safety film is heated toan elevated temperature during that part of the process when it is notunder tension.
 3. A process according to either of the preceding claimscharacterised in that the flexible safety film is polyvinylbutyral.
 4. Aprocess according to any of the preceding claims characterised in thatthe flexible plastic carrier film is polyethylene terephthalate.
 5. Aprocess according to claim 4 characterised in that the polyethyleneterephthalate has an infra red reflecting coating on at least onesurface.
 6. A process according to any of the preceding claimscharacterised in that at least two separate films of flexible safetyfilm are used to encapsulate the flexible plastic carrier film.
 7. Aprocess according to any of claims 1 to 5 characterised in that a singleflexible safety film is pressed with a single flexible carrier film toform a composite film.
 8. A process according to any of the precedingclaims characterised in that the composite safety film is formed bypassing the flexible safety film or films and the flexible carrier filmthrough a nip roller.
 9. A process according to any of claims 1 to 6 and8 characterised in that two separate films of flexible safety film aredrawn from two separate feed rollers under substantial identicalconditions and are incorporated into the composite film undersubstantially identical conditions.
 10. A process according to any ofthe preceding claims characterised in that the temperature of theflexible safety film or films at the point at which they areincorporated into the composite film is in the range 30° C. to 120° C.11. A process according to any of claims 1 to 10 characterised in thatthe temperature of the flexible safety film is raised to one which isabove the bonding temperature during that part of the process when it isnot under tension.
 12. A process for production of a laminated glazingcomprising at least two sheets of glass having at least one interlayerbetween them said interlayer comprising at least one layer of a flexiblesafety film and at least one layer of a plastic carrier film whichcomprising forming a prelaminate assembly comprising said sheets ofglass having said interlayer between them and heating said assembly inan autoclave so as to produce a laminated glass product which ischaracterised in that said interlayer has been produced by a processaccording to any of claims 1 to
 11. 13. A composite film, characterisedin that said film has been produced by a process according to any ofclaims 1 to
 11. 14. A laminated glazing characterised in that itcomprises at least one composite film according to claim 13.